Critical heat flux (CHF) was measured for a water-cooled micro-channel heat sink containing 21 parallel 215 X 821 pm channels. Tests were performed with deionized water over a mass velocity range of 86-368 kg/m(2)s, inlet temperatures of 30 and 60 degreesC, at an outlet pressure of 1.13 bar. As CHIF was approached, flow instabilities induced vapor backflow into the heat sink's upstream plenum, which significantly altered the coolant temperature at the channel inlets. The backflow negated the advantages of inlet subcooling, resulting in a CHF virtually independent of inlet temperature but which increases with increasing mass velocity. Due to the vapor backflow and other unique features of parallel micro-channels, it is shown previous correlations that are quite accurate at predicting CHF for single mini-channels are unsuitable for micro-channel heat sinks. Using the new heat sink water CHF data as well as previous data for R-113 in heat sinks with multiple circular mini- and micro-channels, a new CHF correlation is proposed which shows excellent accuracy in predicting existing heat sink data. (C) 2004 Elsevier Ltd. All rights reserved.